What is the ideal prop size verses Plane width?

When designing a Plane. What would be a good ratio between prop size and plane width?

Optimal prop diameter depends solely on a propeller shape, blade count, design speed and motor parameters. You should also check low speed thrust for takeoff performance.

For example I have a plane (not flown yet) with 3m wingspan and 2 folding props that are over 40cm. IIRC 15x10 but with slightly larger hubs. They are (IMHO too) large and were chosen to maximize the range.

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In general larger props can be more efficient, but also have more risk of hitting the ground or (in a twin) the fuselage. Also, you may not find motors in the right Kv for a particular prop diameter/pitch and battery voltage.

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Exactly, in case of the plane I am working on my predecessors decided to use folding props to address hitting the ground. Now we need guide rods to make sure props won’t get stuck on the wings.